In recent years, the need for a sensor system has increased due to requirement for personal authentication to avoid security risks involved with the spread of networks, the trend of automatic operation of automobiles, or the spread of so-called “Internet of Things.” There are various types of sensors and various types of information to be detected. As one means, there is a method of irradiating an object with light from a light source and obtaining information from reflected light. For example, a pattern authentication sensor, an infrared radar, and the like are examples of the sensors.
As a light source for each sensor, one having wavelength distribution, brightness, and spread corresponding to the application is used. As the wavelength of light, visible light to infrared rays are often used, and in particular, since the infrared rays have such characteristics that the infrared rays are not easily influenced by external light, are invisible, and are able to observe somewhat inside an object, the infrared rays are widely used. In addition, as the type of the light source, an LED light source, a laser light source, or the like is often used. For example, a laser light source with less spread of light is suitably used for detecting a distant place, whereas an LED light source is suitably used for the case of detecting a relatively near place or for irradiating an area having a certain degree of spread.
By the way, the size and shape of the irradiation area as a target are not necessarily coincident with the spread (profile) of light from the light source, and in that case, it is necessary to shape the light with a diffusion plate, a lens, a shielding plate, or the like. Recently, a diffusion plate, called a Light Shading Diffuser (LSD), which is capable of shaping light to some extent, has been developed.
In addition, a Diffractive Optical Element (DOE) may be another example of means for shaping light. It is an application of a diffraction phenomenon, which occurs when light passes through a place where materials with different refractive indices are arranged periodically, to such an element. The DOE is designed basically for light having a single wavelength. However theoretically, it is possible that the DOE shapes light into an almost desired shape. In addition, in the above-described LSD, the light intensity within the irradiation area has a Gaussian distribution, whereas in the DOE, uniformity of the light distribution within the irradiation area can be controlled. Such characteristics of the DOE are advantageous in terms of high efficiency resulting from suppressing irradiation of unnecessary areas and miniaturization of devices due to reduction in the number of light sources, and the like (refer to, for example, Patent Document 1).
In addition, the DOE is compatible with any of parallel light sources such as lasers and diffusion light sources such as LEDs. The DOE may also be applied to a wide range of wavelengths from ultraviolet light, visible light to infrared light.
The DOE requires microfabrication in the order of nm, and in particular, in order to diffract light having a long wavelength, it has been necessary to form a fine shape with a high aspect ratio. For this reason, an electron beam lithography technique using an electron beam has been conventionally used for manufacturing the DOE. For example, a desired DOE can be obtained by depositing a hard mask or a resist on a quartz plate that is transparent in a range of the ultraviolet light to near infrared light, after that, drawing a predetermined shape on the resist by using an electron beam, performing resist development, dry etching of the hard mask, and dry etching of the quartz to form a pattern on the surface of the quartz plate, and after that, removing the hard mask.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2015-170320